Recycle catalytic hydrocracking



y 1962 F. G. CIAPETTA ETAL 3,046,219

RECYCLE CATALYTIC HYDROCRACKING 4 Sheets-Sheet 3 Filed May 14, 1959VOLUME CYCLE STOCK //V CHARGE IOO aoT

4 2 N #GQ: 5G is VOLUME CYCLE STOCK //V CHARGE //7)/en/0r$ Fran/r CCfape/fa Harry LCoonmdf IOO CO/VVE/FS/O/V, I/OL. (/00 RECYCLE) Af/omey3,546,219 Patented July 24, 1962 free 3,046,219 RECYCLE CATALYTICHYDROCRAC-KING Frank G. Ciapetta, Silver Spring, Md, and Harry L.

Coonradt, Woodbury, and William E. Garwood, Haddonfield, N.J., assignorsto Socony Mobil Oil Company, Inc., a corporation of New York Filed May14, 1959, Ser. No. 813,247 7 Claims. (Cl. 208-111) This application is acontinuation-in-part of application Serial Number 491,256, filed March1, 1955, now abancloned.

This invention relates to the catalytic cracking of petroleumhydrocarbon stocks. It is more particularly concerned with a processwherein relatively high boiling hydrocarbon fractions are converted, inthe presence of platinumor palladium-containing catalysts, into valuableproducts.

As is well-known to those familiar with the art, the major products of acracking operation are dry gas, butanes, pentanes, light naphtha, heavynaphtha, and cycle stock (boiling at temperatures higher than about 390F). The light naphtha fraction usually has a rela tively high octanenumber (about 90-92, Fl+3 cc. TEL). On the other hand, the heavy naphthafraction, particularly that which is obtained by cracking in thepresence of hydrogen, has a relatively low octane numher (about 70-80).Accordingly, in order to produce a finished gasoline having a relativelyhigh octane number, it has been the practice to blend the heavy naphthafraction with the light naphtha fraction and also with butanes andpentanes in amounts limited by the mam'murn permissible vapor pressure.There is, however, a steadily increasing demand for higher octanegasolines (about 95 and higher).

As those skilled in the art will readily appreciate, such octanerequirements cannot be met by the aforedescribed conventional blendingoperations. Accordingly, the rela tively low octane heavy naphthafraction has been sub-. jected to reforming operations. As theincreasing demand for higher octane gasolines must be satisfied largelyby reforming, instead of by blending, there is a greater de mand forheavy naphtha fractions that can be reformed and a correspondinglylesser demand for light naphtha fractions that can be used for blendingpurposes. The light naphtha fraction is not usually subjected to areforming operation because it produces excessive amounts of dry gas,coke, etc. The yield of gasoline obtained by reforming light naphtha,therefore, is prohibitively small. Accordingly, it will be appreciatedthat a cracking operation that will produce greater amounts of the heavynaphtha fraction and lesser amounts of light naphtha fractions is highlydesirable.

As is Well-known to those skilled in the art, much of the material thatis obtained in a cracking operation and that boils at temperatureshigher than about 390 F. is a fuel oil suitable for use in Dieselengines and in domestic heating units. The burning efiiciency of thisfuel oil is dependent upon its diesel indexthe higher the diesel index,the better the burning ehiciency of the fuel. It is highly desirable,therefore, to obtain a fuel oil having the highest possible dieselindex.

In copending application Serial Number 825,0l6 tiled July 6, 1959, nowUS. 2,945,806, which was a continuation-in-part of application SerialNumber 418,166, filed on March 23, 1954, now abandoned, which is acontinuation-in-part of application Serial Number 351,151, filed onApril 27, 1953, and now abandoned, there is disclosed a once-throughprocess for cracking high boiling hydrocarbon fractions in the presenceof hydrogen and of catalysts comprising metals of the platinum andpalladium series supported upon synthetic mixed oxide carriers.

'oil having a higher diesel index.

This process can be operated to produce substantially gasoline alone orfuel oil alone, or both. At intermediate conversion levels (levels atwhich both naphtha and fuel oil are made), a good product distributionis obtained. The amount of dry gas is relatively small and the amountsof butanes and of pentanes produced are not in excess of those requiredto produce 10-pound R.V.P. (Reid vapor pressure) gasoline. Substantialyields of light and heavy naphtha are obtained and the diesel index ofthe fuel oil is relatively high. In view of the foregoing, however, theprocess would be more eflicient and its performance would be moredesirable, if it could be operated to produce even less dry gas, lesslight naphtha, more heavy naphtha, and a fuel oil having a still higherdiesel index.

It has now been found that the cracking process described in theaforementioned applications can be operated in a manner that willproduce one or more of the following results: lower dry gas yield, loweryield of light naphtha, greater heavy naphtha production, and a fuel ithas been discovered that these results can be obtained by a recycleoperation in which the recycle ratio is controlled. As will be discussedhereinafter, however, all these desirable results are not obtained withevery charge stock.

Accordingly, it is an object of this invention to provide a method forobtaining more eflicient product distribution in a cracking operation.Another object is to provide a process for cracking relatively highboiling hydrocarbon fractions that will produce, as compared with aoncethrough operation. one or more of the following results: lower drygas yield, lower yield of light naphtha, greater heavy naphthaproduction, and a fuel oil having a higher diesel index. A specificobject is to provide a process for cracking hydrocarbon charge stocks inthe presence of hydrogen and of catalysts comprising metals of theplatinum and palladium series deposited upon a synthetic composite oftwo or more refractory oxides which has a relatively high crackingactivity, that will produce one or more of the aforementioned results.Otther objects and advantages of this invention will become apparent tothose skilled in the art from the following detailed descriptionconsidered in conjunction with the drawings, in which FIGURE 1 presentsa series of curves showing graphically the relationship between thetemperature and the volume percent conversion into products boiling attemperatures lower than about 390 F. (LOO-recycle), and the percentconversion into fuel oil, obtained by cracking a typical, heavy gas oilin the presence of hydrogen and of a platinum-containing catalyst, bothwith and without the recycle operation of this invention;

FiGURE 2 presents a series of curves showing graphically therelationship between the volume percent conversion into products boilingat temperatures lower than about 390 F. GOO-recycle) and the Weightpercent yield of dry gas obtained by cracking a typical, heavy gas oilin the presence of hydrogen and of a platinum-containing catalyst, bothwith and Without the recycle operation of this invention;

FIGURE 3 presents a series of curves showing graphically therelationship between the volume percent con version into productsboiling at temperatures lower than about 390 F. (IOU-recycle) and thevolume percent yield of light naphtha obtained by cracking a typical,heavy gas oil in the presence of hydrogen and .of a platinumcontainingcatalyst, both with and without the recycle operation of this invention.

FIGURE 4 presents a series of curves showing graphically therelationship between the volume percent conversion into products boilingat temperatures lower than about 390 F. '(l00erecyole') and the volumepercent yield of heavy naphtha obtained by cracking a typical,

heavy gas oil in the presence of hydrogen and of a platinum-containingcatalyst, both with and without the recycle operation of this invention;

FIGURE presents a series of curves showing graphically the relationshipbetween the volume percent conversion into fuel oil, the materialboiling at temperatures higher than about 390 F., and the diesel indexof the fuel oil obtained by cracking a typical, heavy gas oil in thepresence of hydrogen and of a platinum-containing catalyst, both withand without the recycle operation of this invention;

FIGURE 6 presents the graphic relationship between the temperature andthe volume percent cycle stock in the charge obtained by cracking acharge comprising a typical, heavy gas oil and varying amounts of cyclestock, in the presence of hydrogen and of a platinum-containing catalystand under conditions that produce 50 percent conversion into productsboiling at temperatures lower than about 390 F. (100-recycle);

FIGURE 7 presents the graphic relationship between the weight percentyield of dry gas and the volume percent cycle stock in the chargeobtained by cracking charge stock-s comprising a typical, heavy gas oiland varying amounts of cycle stock, in the presence of hydrogen and of aplatinum-containing catalyst and under conditions that produce 50percent conversion into products boiling at temperatures lower thanabout 390 F. (100- recycle);

FIGURE 8 presents the graphic relationship between the volume percentcycle stock in the charge and the diesel index of the fuel oil obtainedby cracking charge stocks comprising a typical, heavy gas oil andvarying amounts of cycle stock, in the presence of hydrogen and of aplatinum-containing catalyst and under conditions that produce 50percent conversion into products boiling at temperatures lower thanabout 390 F. (100-recycle);

FIGURE 9 presents a series of curves showing graphically therelationship between the volume percent conversion into products boilingat temperatures lower than about 390 F. (100-recycle) and the weightpercent yield of dry gas obtained by cracking a typical, light gas oilin the presence of hydrogen and of a platinum-containing catalyst, bothwith and without the recycle operation of this invention;

FIGURE 10 presents a series of curves showing graphically therelationship between the volume percent conversion into fuel oil, thematerial boiling at temperatures higher than about 390 F., and thediesel index of the fuel oil obtained by cracking a typical, light gasoil in the presence of hydrogen and of a platinum-containing catalyst,both with and without the recycle operation of this invention;

FIGURE 11 presents a series of curves showing graphically therelationship between the volume percent conversion into products boilingat temperatures lower than about 390 F. (100-recycle) and the weightpercent yield of dry gas obtained by cracking a typical, coker gas oilin the presence of hydrogen and of a platinum-containing catalyst, bothwith and without the recycle operation of this invention; and

FIGURE 12 presents a series of curves showing graphically therelationship between the volume percent conversion into fuel oil, thematerial boiling at temperatures higher than about 390 F., and thediesel index of the fuel oil obtained by cracking a typical, coker gasoil in the presence of hydrogen and of a platinum-containing 4 F., a 50percent point of at least about 500 F., and an end boiling point of atleast about 600 F., and boiling substantially continuously between saidinitial boiling point and said end boiling point, with a cycle stockobtained from cracking said fraction, in which the volumetric ratio ofsaid cycle stock to said fraction varies between about 0.1:1,respectively, and 10:1, respectively, with a catalyst comprising betweenabout 0.05 percent and about 20 percent, by Weight of the catalyst, ofat least one metal of the platinum and palladium series deposited upon asynthetic composite of at least two refractory oxides, said compositehaving an activity index of at least about 25, in the presence ofhydrogen in amounts, expressed in molar ratio of hydrogen to hydrocarboncharge, within the range of about 2 to about 80, at a pressure abovepounds per square inch gauge, at a liquid hourly space velocity withinthe range about 0.1 to about 10, and at a temperature above about 500 F.

Throughout the specification and the claims, the term conversion isintended to be a generic term for the amount of products boiling attemperatures lower than about 390 F. (100-recycle), of gasoline, or offuel oil obtained in the process. It is expressed in terms of the volumepercent of the initial charge which is transformed in the process. Theamount of product boiling at temperatures lower than about 390 F. isobtained by subtracting the volume percent of cycle stock from 100percent, i.e., from the initial volume of the charge. The expression(100- ecycle) is an abbreviation for 100 percent minus the volumepercent recycle. Dry-gas refers to the methane, ethane, propane, andethylene and propylene produced in a cracking process, expressed interms of Weight percent of the initial charge. Light naphtha is theproduct that boils between about F. and about F. The reavy naphtha isthe product that boils between about 170 F. and about 390 F. The dieselindex of the fuel oil is a function of the A.P.I. gravity and theaniline number, as defined by Becker et al. in the S.A.E. Journal(Transactions), vol. 35, No. 4, p. 377. The cracking activity of acarrier is expressed in terms of the percent, by volume, of a standardhydrocarbon charge which is cracked, under specific operatingconditions, in the Cat. A test. This test is described by Alexander andShimp in National Petroleum News, 36, page R-537 (August 2, 1944). Theunit for rating the cracking activity of a material is called theactivity index (AL).

The catalysts utilizable herein are those described in US. 2,945,806, acontinuation-in-part of Serial Number 418,166, filed on March 23, 1954,now abandoned, which in turn was a continuation-in-part of applicationSerial Number 351,151, filed on April 27, 1953, now abandoned. Briefly,these catalysts comprise between about 0.05 percent, by Weight, andabout 20 percent, by weight, of the final catalyst, preferably betweenabout 0.1 percent and about 5 percent, by weight, of the metals of theplatinum and palladium series, i.e., those having atomic numbers of44-46, inclusive, 76-78, inclusive, supported upon synthetic compositesof two or more refractory oxides. The carrier is a synthetic compositeof two or more oxides of the metals of groups 11A, IILB and IVA and B ofthe periodic arrangement of elements [1. Chem. Ed., 16, 409 (1939)].These synthetic composites of refractory oxides must have an activityindex of at least about 25. They can also contain halogens and othermaterials which are known in the art as promoters for crackingcatalysts, or small amounts of alkali metals that are added for thepurpose of controlling the activity index of the carrier. Non-limitingexamples of the composites contemplated herein include silica-alumina,silica-zirconia, silica-alumina-zirconia, silica-alumina thoria,alumina-bori-a, silica-magnesia, silica-alumina-magnesia,silicaah1min-aflu0rine, and the like. The preferred support is asynthetic composite of silica and alumina containing scream betweenabout 1 percent, by weight, and about 90 percent, by weight, of alumina.These synthetic composites of two or more refractory oxides can be madeby any of the usual methods known to those skilled in the art ofcatalyst manufacture. Examples of methods of preparing them are setforth in US. 2,945,806.

The following example illustrates a method of preparing a'plat-inumcontaining catalyst utilizable in the process of thisinvention:

EXAMPLE 1 A synthetic silica-alumina carrier or support containingpercent by weight alumina was prepared by mixing an aqueous solution ofsodium silicate (containing 158 g. per liter of silica) with an equalamount of an aqueous acid solution of aluminum sulfate containing 39.4g. Al (SO and 28.6 g. concentrated H 80 per liter. This mixture ofsolutions was dropped through a column of oil, wherein gelation of thehydrogel was effected in bead form. The bead hydrogel was soaked in hotwater (about 120 F.) for about 3 hours. The sodium in the hydrogel wasthen removed by exchanging the gel with an aqueous solution of aluminumsulfate [1.5 Al (SO by weight] containing a small amount (0.2 percent byweight) of ammonium sulfate. The thus-exchanged hydrogel head waswater-washed. Then, it was driedin superheated steam (about 280-340 F.)for about 3 hours and, finally, calcined at 1300 F. under a low pertialpressure of steam for about 10 hours.

The silica-alumina beads were then crushed to pass through -a l4-meshscreen and the material retained on a -mesh screen (-UJS. standardscreen series) was used for catalyst preparation. Portions of thecrushed, calcined carrier were then barely covered with aqueoussolutions of chloroplatinic acid, of concentrations suilicient toproduce the desired amount of metal in the finished catalyst. The excesssolution was removed by centrifuging. The thus-impregnated carrier wasthen dried at 230 F. for 24 hours. The catalyst was treated withhydrogen for four hours at 400 F. Then, it was activated in hydrogen for16 hours before it was used. The catalyst thus prepared contained 0.47percent platinum, by weight of the catalyst, and the silica-aluminacarrier had an activity index of 46.

The charge stocks utilized in the process of this invention comprisemixtures of virgin stocks with cycle stocks. As used throughout thespecification and claims, the term virgin stoc means a hydrocarbonfraction that has not been subjected to the process of this invention orto the process described in the aforementioned copending application.Likewise, throughout the specification and the claims, the terms cyclestock" or recycle refer to hydrocarbon firaotions boiling attemperatures higher than about 390 F. that have been subjected to theprocess of this invention and are returned to the reaction zone.

The virgin stock utilizable herein are hydrocarbon fractions, having aninitial boiling point of at least about 400 F., a 50 percent point of atleast about 500 F. and an end boiling point of at least about 600 F. andboiling substantially continuously between said initial boiling pointand said end boiling, point. Such charge stocks inelude gas oils,residual stocks, refractory cycle stocks from conventional cracking,whole topped crudes, and heavy hydrocarbon fractions derived by thedestructive hydrogenation of coal, tars, pitches, asphalts, etc., suchas, for example, middle. oil. a

As is well-known to those skilled in the art, the distillation of higherboiling petroleum fractions (those boiling at temperatures higher thanabout 750 F.) must he carried out under vacuum, in order to avoidthermal cracking. Throughout the specification and in the claims,however, the boiling temperatures are expressed in terms of the boilingpoint at atmospheric presure. In other words, in all instances, theboiling points of fractions distilled under vacuum have been correctedto the boiling points at atmospheric pressure.

As is well-known to those familiar with the art, the term gas oil is abroad, general term that covers a variety of stocks. Throughout thespecification and in the claims, the term, unless further modified,includes any fraction distilled from petroleum which has an initialboiling point of at least about 400 F., a 50 percent point of at leastabout 500 F., and an end boiling point of at least about 600 F., andboiling substantially continuousiy between the initial boiling point andthe end boiling point. The portion which is not distilled is consideredresidual stock. The exact boiling range of a gas oil, therefore, will bedetermined by the initial distillation temperature (initial boilingpoint), the 50 percent point, and by the temperature at whichdistillation is cut off (end boiling point).

in practice, petroleum distiliations have been made under vacuum up totemperatures as high as 11001200 F. (corrected to atmospheric pressure).Accordingly, in the broad sense, a gas oil is a petroleum fraction whichboils substantially continuously between two temperatures that establisha range falling within from about 400 F. to about 11001200 F., the 50percent point being at least about 500 F. Thus, a gas oil could boilover the entire range 4001200 F. or it could boil over a narrower range,e.g., 500-900 F.

The gas oils can be further roughly subdivided by overlapping boilingranges. Thus, a light gas oil boils between about 400 F. and about600650 F. A medium gas oil distills between about 600 650 F. and about700- 750 F. A heavy gas oil will boil between about 600- 650 F. andabout 800-900 F. A gas oil boiling between about 800850 F and about1100-1200 F. is sometimes designated as a vacuum gas oil. It must beunderstood, however, that a gas oil can overlap the foregoing ranges. Itcan even span several ranges, i.e., include, for example, light andmedium gas oils.

As mentioned hereinbefore, a residual stock is any fraction which isnotdistilled. Therefore, any fraction, regardless of its initial boilingpoint, which includes all the heavy bottoms, such as tars, asphalts,etc., is a residual fraction. Accordingly, a residual stock can be theportion of the crude remaining undistilled at 1100- 1200 F., or it canbe made up of a gas oil fraction plus the portion undistilled at1100-1200 F. A whole topped crude, as the name implies, is the entireportion of the crude remaining after the light ends (the portion boilingup to about 400 F.) have been removed by distillation. Therefore, such afraction includes the entire gas oil fraction (400 'F. to 11001200 F.)and the undistilled portion of the crude petroleum boiling above1100l200 F. If it is desired, the residual fractions and the wholetopped crude can be deasphalted by any means known to the art. Suchtreatment, however, is not necessary for charge stocks intended for usein the process of this invention.

The refractory cycle stocks are cuts of conventionally cracked stocksWhich boil above the gasoline boiling range, usually, between about 400F. and about 850 F. The refractory cycle stocks can be charged to theprocess of this inventionin conjunction with a fresh petroleum chargestock, or they can be charged alone to the process. The process of thisinvention is particularly adaptable to the cracking of sulfur-containingcharge stocks. The catalysts utilizable in the process of thisinvention, quite unexpectedly, are not deactivated by sulfur compounds,under the conditions of the process.

The presence of even relatively small amounts of nitrogen compounds inthe charge stock causes the degree of conversion which can be attainedat any given temperature to decrease. Thus, the higher the nitrogencontent the greater will be the temperature needed to effect a givenamount of conversion. Since higher temperatures spacers are usuallyassociated with higher dry gas production, there will usually be agreater quantity of dry gas produced when converting high nitrogenstocks to the same degree as lower nitrogen stocks.

It is, therefore, prefer-red for optimum operations that the nitrogencontent of the charge stock be below about 0.1 percent by weight andstill more preferably below about 0.08 percent by Weight. if desired,the nitrogen content of any charge stock may be reduced prior to itssupply to the process of this invention by conventional treatment, suchas acid treatment, extraction with propane or hydro-genolysis under veryhigh pressure in contact with catalysts such as molybdenum or tungstenoxide, nickel sulfide, tungsten sulfide, cobalt molybdate, cobalttungstate, etc.

It must be understood that all the foregoing types of virgin stock donot produce similar results when subjected to the process of thisinvention. In other words, all'charge stocks do not produce all, or eventhe same advantageous results as compared with once-through op eration,viz., lower operating temperatures, lower dry gas yield, lower lightnaphtha yield, higher heavy naphtha yield and a fuel oil having a higherdiesel index. Some charge stocks, notably those having initial boilingpoints of the order of about 600650 F. or higher, and more particularlystraight-run gas oils having such initial boiling points, do, indeed,produce all the foregoing advantageous results. On the other hand, otherstocks, particularly light gas oils, produce only a few of them. Allvirgin charge stocks that are admixed with cycle stock do produce,however, fuel oils having higher diesel indexes.

The variation in results obtained with different charge stocks willbecome apparent to those skilled in the art from the following examples:

EXAMPLE 2 The charge stocks used in this example were a mixturecontaining five parts of cycle stock derived from Kuwait gas oil and onepart of virgin Kuwait gas oil, a mixture containing one part of cyclestock derived from Kuwait gas oil and one part of Kuwait gas oil, andthe virgin Kuwait gas oil alone. These charge stocks had the followingproperties:

Each mixture was subjected to cracking in the presence of hydrogen andof the catalyst described in Example 1 after the latter had reachedequilibrium, i.e., had been in continuous operation for more than fivedays. The hydrogen pressure used was 1000 p.s.i.g., the liquid hourlyspace velocity was 0.5, and the molar ratio of hydrogen to oil was 40.

For purposes of comparison, portions of the virgin Kuwait gas oil,without recycle, were subjected to cracking under the same conditions,at various temperatures, and in the presence of the same catalyst usedin cracking the mixtures of cycle stock with virgin Kuwait gas oil. Thepertinent data are set forth in Table I.

Q Table 1 Charge s11 blend 1:1 blend cycle plus cycle Virgin Kuwait gasoil virgin plus Kuwait virgin Kuwait 0 Operating conditions:

emp, r 743 785 793 694 710 Pros-sure, p s .1 1,000 1, 000 1, 000 1,0001,000 LrlSV 0.5 0.5 0.5 .5 0.5 111/011 ratio 40 40 40 Results:

Conversion, vol. percent. 28. 1 52. 1 60. 2 44. 4 46. 9 Dry gas, weightpercent 2. 2 3. 8 5. 9 1. 6 1. 8 14) Butancs, vol. percent 4. 8 11. 014. 8 6. l 8. 8 Pent-e es vol. percen 4. 3 9.1 11.5 5. 4 6. 9 in, vol.percent 3. 7 7. 6 0. 7 4. 8 4. 8

23. 6 36. 3 -17. 4 38. 3 37. 2 Fuel oil, vol. percent... 71. 9 47. b 30.8 55.6 53.1 Diesel index of fuel oil, 61.4 71.0 74. 2 68.8 20

, The curves shown in FIGURES 1 through 5 are based upon the data setforth in Table I. In FIGURE '1, curve 1 shows the relationship betweenthe temperature and the volume percent conversion into products boilingat temperatures lower than about 390 F. 100-recycle) in the case inwhich the Kuwait gas oil alone was subjected to cracking without usingrecycle. Curves 2 and 3, respectively, show a similar relationshipobtained by cracking the gas oil, using recycle in ratios of 1:1 and5:1, respectively. It will be noted that much lower temperatures arerequired to achieve cracking at any conversion level when recycleoperation is used than in the operation in which virgin feed alone isemployed. It is to be noted also that with the higher recycle ratios,lower temperatures are required.

in FIGURE 2, curve 4 shows the relationship between the weight percentyield of dry gas and the volume per cent conversion into productsboiling at temperatures lower than about 390 F. (100-recycle) in thecase in which the virgin gas oil alone was used. Curve-s 5 and 6,respectively, show similar relationships obtained in cases in whichrecycle ratios of 1:1 and 5:1, respectively, were used. It is to benoted that at the same conversion 0 le el, the amount of dry gasproduced in the recycle operation of this invention is considerably lessthan that produced in a once-through operation.

In FIGURE 3, curve 7 shows the relationship between the volume percentyield of C light naphtha and the volume percent conversion into productsboiling at temperatures lower than about 390 F. (IOU-recycle) obtainedin the case in which the virgin gas oil alone was subjected to cracking.Curve 8 shows a similar relationship obtained in the cases in which therecycle operation .1 of this invention was used. It will be noted thatat the same conversion level, the amount of light naphtha produced incycle operation is appreciably smaller. As has been mentionedhereinbefore, this is of considerable importance because this lightnaphtha does not have a 9 sufficiently high octane rating to meet thedemand for high octane gasolines and, generally, it is not subjected toa reforming operation.

In FIGURE 4, curve 9 shows the relationship between the volume percentyield of heavy naphtha and the ,5 volume percent conversion intoproducts boiling below about 390 F. (l00-recycle) in the case in whichthe virgin Kuwait gas oil alone was subjected to cracking. Curves 10 and11, respectively, show similar relationships obtained in the cases inwhich recycle ratios of 1:1 and 5:1, respectively, were employed. Itwill be apparent that, as the recycle ratio is increased, the yield ofheavy naphtha at any conversion level is higher. It has been foundfurther that, from the standpoint of ease of conversion into high octanegasoline by reforming, the qualrty of this naphtha is high and that itremains subof decrease in dry gas yield is less.

stantially so at any conversion level. As this stock is one that isreadily reformed into high octane gasoline, an increase in its yield ishighly desirable.

In FIGURE 5, curve 12 shows the relationship between the diesel index ofthe fuel oil and the volume percent conversion into fuel oil boiling attemperatures higher than about 390 F. obtained by cracking the virginKuwait gas oil alone. Curves 13 and 14 present similar relationshipsobtained by using recycle ratios of 1:1 and 5:1, respectively. Theconsiderably higher diesel indexes obtained using the process of thisinvention over those that can be obtained with the virgin stock alone isreadily apparent. As is well-known to those skilled in the art, theburning qualities of a fuel used for diesel engines or for heatingimprove as the diesel index increases.

In view of the foregoing discussion, it is to be noted that, when theheavier charge stocks are used in the process of the present invention,all the advantageous results listed hereinbefore are obtained. Thispermits much more flexibility in operation. Thus, not only can therelative amounts of desirable and of undesirable products be controlled,but lower temperatures can be used. This means, of course, that the veryheavy stocks, such as residual stocks, can be cracked at temperaturessufficiently low that coking and dry gas production are cut downconsiderably, resulting in increased catalyst life. The product boilingat temperatures higher than about 390 F. has a lower end boiling pointthan that of the charge and has a high diesel index. Accordingly, theheavier charge stocks (particularly straight-run charge stocks) havingan initial boiling point of 600650 F. can be converted by the process ofthis invention substantially completely into gasoline having goodreforming charge stock characteristics and into fuel oil having a highdiesel index.

The ratio of cycle stock to virgin stock in the charge for the processof this invention must vary between well established limits, if theadvantages enumerated hereinbefore are to be realized. This will beapparent from the curves set forth in FIGURES 6, 7 and 8, which havebeen derived from FIGURES 1, 2 and 5, respectively.

The curve in FIGURE 6 shows'the relationship between the volume percentcycle stock in the charge and the temperature at the level of 50 percentconversion into products boiling at temperatures lower than about 390 F.(100-recycle). This curve was obtained by determining, from the curvesin FIGURE 1, the temperature required for 50 percent conversion usingvarying amounts of cycle stock (83.3%) in the feed. It will be notedthat when the volume percent of cycle stock in the charge is about 50percent (1:1 ratio), the temperature required to produce 50 percentconversion is about 60 F. lower than that required for virgin stockalone. On the other hand, at a ratio of about :1, the decrease in thetemperature requirement is about F. more. At higher recycle ratios, thedecrease in temperature requirement is correspondingly less.

The curve in FIGURE 7 shows the relationship between the volume percentcycle stock in the charge and the weight percent dry gas, at the levelof 50 percent conversion into products boiling at temperatures lowerthan about 390 F. (IOO-recydle). This curve was derived from FIGURE 2 inthe same manner that the curve in FIGURE '6 was derived from FIGURE 1.It will be noted that the greatest decrease in dry gas yield is obtainedwhen the recycle ratio is increased to about 1:1. As the recycle ratiois further increased, the degree The curve in FIGURE 8 shows therelationship between the volume percent cycle stock in the charge andthe diesel index of the fuel oil at the level of 50 percent conversioninto fuel oil (boiling at temperatures higher than about 390 F). It wasderived from FIGURE 5 in the same manner that the curves in FIGURES 6and a 10 7 were derived from FIGURES 1 and 2, respectively. It

I will be noted that the increase in the diesel index of the fuel oilremains small until the recycle ratio is about 1:1. The curves shown inFIGURES 6, 7 and 8 indicate that very substantial advantages may beobtained in reduction of the temperatures required to effect a givenconversion and in dry gas with recycle ratios as low as 0.121. To alsoobtain an improvement in diesel index of the fuel oil, the ratio ofcycle stock to' virgin stock in the charge ordinarily should be at leastabout 1:1, however. The ratio of cycle stock to virgin stock can be ashigh as 15 :1 or higher. In practice, however, it is preferred to usecycle stock to virgin stock volume ratios varying between about 1:1 -orslightly lower and about 10:1 and higher and, more preferably, betweenabout 1:1' and about 5:1.

As has been mentioned hereinbefore, when light gas oils are cracked,using the cycle operation of this invention, all the aforementionedadvantageous results are not obtained. This will be apparent from thefollowing examples:

EXAMPLE 3 The charge stocks used in this example were a mixture of equalparts,'by volume, of a virgin light East Texas gas oil with a cyclestock derived from the light East Texas gas oil, and the virgin lightEast Texas gas gas oil was cracked in the presence of hydrogen and ofthe same catalyst used in the runs described in Example 2. The run Wascarried out under a hydrogen pressure of 1000 p.s.i.g., at a liquidhourly space velocity of 0.5, and using a hydrogen to oil molar ratio of40. For comparison purposes, the virgin light East Texas gas oil alonewas cracked under the same conditions and at "various temperatures,using the same catalyst. The pertinent data are set forth in Table II.

Table 11 Charge 1:1 blend of cycle plus Virgin East Texas gas virgin oilEast; Texas gas oil Operating conditions:

Temp, F M0 685 700 065 Pressure, F 1,00 1 1,000 1,000 1.000 LHSV .5 0. 50.5 0.5 Hz/oil ratio 40 40 40 40 Results:

Conversion, vol. percent..." 13.3 49. 7 68.2 33. 6 Dry gas, weightpercent 0. 4 2.1 2. 1 0. 5 Butanes, vol. percent 1. 3 5.8 8. 5 ,4. 1Pentanes, v01. percent........ 1.0 4. 5 9. 4 3.3 Lt. naphtha, vol.percent 1.4 4.1 7.8 1. 5 Hvy. naphtha, vol percent 15.2 40.1 56.4 29.5Fuel oil, vol. percent 86. 7 50. 3 31.8 66. 4 Diesel index of fuel oil76. 4 83. 9 I 86. 3 '85. 7

-oils are the charge stocks.

An attempt to show the relationship graphically be tween the volumepercent conversion into products boiling at temperatures lower thanabout 390 F. and the temperature, in the case in which the light EastTexas gas oil alone is cracked and in the case of the cycle operation ofthis invention, produced a single curve, regardless of the type ofoperation. This means of course, that, with respect to the temperatureof operation, the operation of this invention, when light gas oils areused, affords no advantage. Similar results were obtained in an attemptto show the relationship between the volume percent conversion intoproducts boiling at temperatures lower than about'390 F. (lOO-recycle)on the one hand and the volume percent yield of light naphtha and thevolume percent yield of heavy naphtha. The conclusion is, therefore,that there is no advantage gained from the standpoint of a decrease inlight naphtha production or of an increased yield of heavy naphtha byapplying the process of this invention to light gas oils.

There are, however, at least two real advantages, namely, the decreasedproduction of dry gas and the increased diesel index of the fuel oil.This will be apparent from the curves shown in FIGURES 9 and 10. Thesecurves are based upon the data set forth in Table II.

In FIGURE 9, curve 15 shows the relationship between the volume percentconversion into products boiling at temperatures lower than about 390 F.and the weight percent yield of dry gas produced in the case in whichthe "irgin light gas oil alone is cracked in the presence of hydrogenand a platinum-containing catalyst. Curve 16 shows a similarrelationship in the case in which the light gas oil is cracked in theprocess of this invention, using a 1:1 recycle ratio. It will beapparent that in the latter the amount of dry gas produced isconsiderably less, particularly at the higher conversion levels.

In FIGURE 10, curve 17 shows the relationship between the volume percentconversion 'into products boiling at temperatures lower than about 390F. and the diesel index of the fuel oil produced in the case in whichthe light gas oil alone is cracked. Curve 18 shows a similarrelationship in the case in which the light gas .oil is cracked in theprocess of this invention, using a 1:1

recycle ratio. It will be noted that the process of this inventionproduces a fuel oil having a much higher diesel index and in betteryields than can be obtained in a oncethrough operation. Thus, forexample, if it is desired to produce a fuel oil having a diesel index of85, it can be produced in about 68 volume percent yield by the processof this invention, as compared to a yield of only about 40 volumepercent in the case in which no recycle operation is involved. Thismeans, of course, that 28 volume percent more of the desired fuel oilcan be obtained in the present process.

From the data set forth in Table II and the curves presented in FIGURES9 and 10, therefore, it will be apparent to those skilled in the artthat the application of the process of this invention to light gas oilis advantageous. The advantages are real and commercially valuable.

The application of the present process to light gas oils other thanstraight-run light gas oils, such as light coker gas oils, likewise doesnot produce all the advantageous results obtained as in the cases inwhich the heavier gas This will be apparent from the following examples:

EXAMPLE 4 The charge stocks used in this example were a mixture of equalparts, by volume, of a virgin light gas oil produced by coking aMid-Continent residuum with a cycle stock derived from the light cokergas oil, and the virgin light Mid-Continent coker gas oil alone. Thesecharge stocks had the following properties:

hydrogen and 0f the platinum-containing catalyst that was used in theruns described in Example 2. The hydrogen pressure was 1000 pounds persquare inch gauge, the liquid hourly space velocity was 0.5, and thehydrogen to oil molar ratio was 40. For comparison purposes, the virginlight Mid-Continent coker gas oil alone was cracked at varioustemperatures, using the same catalyst and conditions that were used withthe 1:1 blend. The pertinent data for these runs are set forth in TableIII.

Table III Charge 1:1 blend of cycle stock Virgin light Mid-C0ntiandvirgin nent coker gas oil Mid-Continent coker gas oil Conditions:

Temp, F 750 795 780 Pressure, p.s.i.g. 1, 000 1, 000 1,000 1,000 LHSV .50.5 0.5 0. 5 Hz/Oil ratio 40 40 40 40 Results:

Conversion, vol. percent. 25. 8 43. 2 66. 5 62. 5 Dry gas, weightpercent 1. 5 2. 5 4. 2 3. 6 Butanes, vol. percent.-- 2. 6 6. 5 11.8 10.6Pentanes, vol. percent 1. 4 4. 9 10.0 6. 7 Lt. naphtha, vol. percent 2.7 4. 7 9.8 11.2 Hvy. naphtha, v01. percent. 24. 8 35. 5 46. 6 44.1 Fueloil, vol. percent 74. 2 5G. 8 33. 5 37. 5 Diesel index of fuel oil...62.1 61. 8 59. 5 72.6

As was noted in the case of the light East Texas gas oil, the cycleoperation with the light coker gas oil did not produce all theadvantageous results listed hereinbefore. Thus, at any given conversionlevel, the temperature, the yield of light naphtha, and the yield ofheavy naphtha were substantially the same, both in once-throughoperation and in the cycle operation. There were, however, substantialadvantages from the standpoint of lowered dry gas yield and higherdiesel index of the fuel oil, in the case in which the cycle operationwas used. This will be apparent from the curves in FIGURES 11 and 12.These curves are based upon the data set forth in Table III.

In FIGURE 11, curve 19 shows the relationship between the volume percentconversion into products boiling at temperatures lower than about 390 F.l00-recycle) and the Weight percent of dry gas produced when the virginlight Mid-Continent coker gas oil was cracked in the presence of theplatinum-containing catalyst. Curve 20 shows a similar relationshipobtained in the case in which the 1:1 blend of cycle stock and virgincoker gas oil was cracked. It will be apparent that considerably lessdry gas is produced in the case in which the cracking is carried out inaccordance with this invention.

In FIGURE 12, curve 21 shows the relationship between the volume percentconversion into products boiling at temperatures lower than about 390 F.(lOO-recycle) and the diesel index of the fuel oil produced in the casein which the virgin light Mid-Continent coker gas oil was cracked in thepresence of the platinum-containing catalyst. Curve 22' shows a similarrelationship obtained in the case in which the 1:1 blend of cycle stockand virgin light coker gas oil was cracked. it will be noted that thediesel index of the fuel oil produced by the cycle operation of thisinvention is much higher than that of the fuel oil produced in aone-through operation.

It will be apparent, therefore, that there are real and distinctadvantages in using the process of this invention. In the case of thelight gas oils, straight-run or coker, two advantageous results areobtained. In the case of the heavier gas oils, on the other hand, thereare a number of advantages over once-through operation, viz., loweroperating temperatures, lower dry gas yield, lower light naphtha yield,higher yield of heavy naphtha, and a higher diesel index in the fueloil.

In so far as is known there is nothing critical about the reactiontemperature at which the process of this invention is carried out and itis believed operable at any temperatures, conventional in the art, atwhich hydrocracking, with a net consumption of hydrogen, will occur. Ofcourse, the reaction temperature should not generally exceed the minimumtemperature at which the charge is converted completely into naphtha,since higher temperatures can only crack the naphtha boiling range.material into lower boiling hydrocarbons, including dry gas. For thereaction to proceed at an acceptable rate, it is usually necessary thatthe reaction temperature exceed about 500 F. Preferably, the reactiontemperature should be within the range about 500 F. to about 825 F. andstill more preferably within the range 650 F. to about 825 F.

Likewise, in so far as is known, there is nothing critical about thehydrogen pressure employed in this invention and it will operate withany hydrogen pressures which etfect hydrocracking with a net consumptionof hydrogen by the charge stock. Very high hydrogen pressures, however,make it necessary to employ very expensive heavy walled reactors. It is,therefore, preferable that the hydrogen pressure be within the range 100to 2500 pounds per square inch gauge (p.s.i.g.).

The liquid hourly space velocity of the reactant employed in thisinvention will generally be within the range about 0.1 to about 10 andpreferably 0.1 to about 4 voltunes of reactant (measured as 60 F.liquid) per volume of catalyst per hour. The molar ratio of hydrogen tohydrocarbon usually will be within the range about 2 to about 80 andpreferably about 5 to about 50.

The process of this invention can be carried out using conventionalapparatus and schemes for eifecting recycle operation in catalyticcracking. As the catalyst remains active over long periods of timebefore it must be regenerated, the operation is advantageously carriedout using a fixed bed of catalyst. Other techniques, however, can beused, such as, the moving bed technique or the fluid technique.

Although the present invention has been described with preferredembodiments, it is to be understood that modifications and variationsmay be resorted to, without departing from the spirit and scope of thisinvention, as those skilled in the art will readily understand. Suchvariations and modifications are considered to be within the purview andscope of the appended claims.

We claim:

1. A process for cracking hydrocarbon fractions that has at least one ofthe following advantages over a oncethrough operation: lower operatingtemperatures, lower dry gas yield, lower light naphtha yield, higherheavy naphtha yield, and a fuel oil having a higher diesel index, whichcomprises contacting a mixture of a hydrocarbon fraction having aninitial boiling point of at least about 400 F., a 50 percent point of atleast about 500 F. and an end-boiling point of at least about 600 F. andboiling substantially continuously between said initial boiling pointand said end-boiling point with a cycle stock which is obtained fromhydrocracking said initial hydrocarbon i4 fraction and which boils attemperatures higher than about 390 F., in which the volumetric ratio ofsaid cycle stock to said hydrocarbon fraction ranges from about 1:1,respectively, to about 10:1, respectively, with a catalyst comprisingabout 0.05 percent to about 20- percent, by weight of the catalyst, ofat least one metal of the platinum and palladium series deposited upon asynthetic composite of oxides ofat leasttwo metals of the groups HA,111B and 1V ofthe periodic arrangement of the elements, having anactivity index of greater than 25, in the presence of hydrogen inamounts, expressed in a molar ratio of hydrogen to hydrocarbon charge,ranging from about 2 to about 80, pressures of about pounds per squareinch gauge to about 2500 pounds per square inch gauge, at a liquidhourly space velocity ranging from about 0.1 to about 10, and attemperatures of about 500 F. to about 825 F.

2. A process for cracking hydrocarbon fractions that has at least one ofthe following advantages over a oncethrongh operation: lower operatingtemperatures, lower dry gas yield, lower light naphtha yield, higherheavy naphtha yield, and a fuel oil having a higher diesel index, whichcomprises contacting a mixture of a petroleum gas oil having an initialboiling point of at least about 400 F., a 50 percent point of at leastabout 500 F., and an end-boiling point of at least about 600 F., andboiling substantially continuously between said initial boiling pointand said end-boiling point, and containing less than about 0.08 percentnitrogen, by weight, with a cycle stock which is obtained fromhydrocracking said gas oil, and which boils at temperatures higher thanabout 390 F, in which the volumetric ratio of said cycle stock to saidgas oil ranges from about 1:1, respectively, to about 5:1, respectively,with a catalyst comprising about 0.1 percent to about 5 percent, byweight of the catalyst, of platinum deposited upon a synthetic compositeof silica and alumina having an activity index of at least about 28, inthe presence of hydrogen in amounts, expressed in a molar ratio ofhydrogen to hydrocarbon charge, ranging from about 5 to about 50,pressures ranging from about 350 pounds per square inch gauge to about2000 pounds per square inch gauge, at a liquid hourly space velocityranging from about 0.1 to about 4, and at temperatures of about 650 F.to about 835 F.

3. A process for hydrocracking hydrocarbon fractions that has at leastone of the following advantages over a once-through operation: loweroperating temperatures, lower dry gas yield, lower light naphtha yield,higher heavy naphtha yield, and a fuel oil having a higher diesel index,which comprises contacting a mixture of a hydrocarbon fraction having aninitial boiling point of at least about 400 F., a 50 percent point of atleast about 500 F. and an end-boiling point of at least about 600 F. andboiling substantially continuously between said initial boiling pointand said end-boiling point with a cycle stock which is obtained fromhydrocracking said initial hydrocarbon fraction, and which boils aboveabout 390 F., in which the volumetric ratio of said cycle stock to saidinitial fraction is Within the range of about 0.1 1, respectively, toabout 10:1, respectively, with a catalyst comprising about 0.05 percentto about 20 percent, by weight of the catalyst, of at least one metal ofthe platinum and palladium series deposited upon a synthetic compositeof oxides of at least two elements of the groups IIA, lIIB and IV of theperiodic arrangement of the elements, having an activity index ofgreater than 25, in the presence of hydrogen under reaction conditionswhich produce a a net consumption of hydrogen during the reaction, whichconditions include a molar ratio of hydrogen to hydrocarbon chargewithin the range of about 2 to about 80, a hydrogen pressure above about100 pounds per square inch gauge, a liquid hourly space velocity withinthe range of about 0.1 to about 10, and temperatures above about 500 F.

4. The process of claim 3 wherein the hydrocarbon 15 fraction has aninitial boiling point of at least about 600 F.

5. The process of claim 3 wherein the volumetric ratio of cycle stock tohydrocarbon fraction contacted is Within the range of about 1:1,respectively, to about 10:1, respectively.

6. The process of claim 3 wherein the activity index of the base of thecatalyst is at least 28.

7. The process of claim 6 wherein the hydrocarbon fraction has aninitial boiling'point of at least about References titted in the file ofthis patent UNITED STATES PATENTS Johnson et a1. July 16, 1957 Boedekeret a1. Dec. 2, 1958 Haensel et a1 Sept. 29, 1959 Ciapetta July 19, 1960

1. A PROCESS FOR CRACKING HYDROCARBON FRACTIONS THAT HAS AT LEAST ONE OFTHE FOLLOWING ADVNTAGES OVER A ONCETHROUGH OPERATION: LOWER OPERATINGTEMPERATURES, LOWER DRY GAS YIELD, LOWER LIGHT NAPHTHA YIELD, HIGHERHEAVY NAPHTHA YIELD, AND A FUEL OIL HAVING A HIGHER DIESEL INDEX WHICHCOMPRISES CONTACTING A MIXTURE OF A HYDROCARBON FRACTION HAVING ANINITIAL BOILING POINT OF AT LEAST ABOUT 400*F., A 50 PERCENT POINT OF ATLEAST ABOUT 500*F.AND AN END-BOILING POINT OF AT LEAST ABOUT 600*F. ANDBOLING SUBSTANTIALLY CONTINUOUSLY BETWEEN SAID INITIAL BOILING POINT ANDSAID END-BOILING POINT WITH A CYCLE STOCK WHICH IS OBTAINED FROMHYDROCRACKING SAID INITIAL HYDROCARBON FRACTION AND WHICH BOILS ATTEMPERATURES HIGHER THAN ABOUT 390*F., IN WHICH THE VOLUMETRIC RATIO OFSAID CYCLE STOCK TO SAID HYDROCARBON FRACTION RANGES FROM ABOUT 1:1,RESPECTIVELY, TO ABOUT 10:1, RESPECTIVELY, WITH A CATALYST COMPRISINGABOUT 0.05 PERCENT TO ABOUT 20 PERCENT, BY WEIGHT OF THE CATALYST, OF ATLEAST ONE METAL OF THE PLATI-